Add ability to specify npm path separated from Node.exe path

description

At the moment if you update npm with:

npm install -g npm

you'll find that NTVS still uses whatever version of npm came bundled in the last Node MSI you installed.

The reason for this is that npm is installed in the node directory outside of the global package store, and the node directory will appear first in your PATH environment variable. Also, if installed for all users, it's part of the system PATH, not the user
PATH. This also means that you end up executing the old npm from the command line, which isn't something we can do anything about.

If we allow users to (optionally) specify the location of npm separately from Node.exe this will fix the problem.

We could even infer it based on the location of the global package store and the presence (or absence) of a newer npm package.